1. Field of the Invention
The present invention relates to a thermosensitive recording material comprising as the main components a leuco dye serving as a coloring agent and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto, and a particular thermosensitivity improving agent.
2. Discussion of Background
There are conventionally proposed various recording materials which utilize the coloring reaction between a colorless or light-colored leuco dye and a color developer capable of inducing color formation in the leuco dye upon application of heat or pressure thereto when brought into contact with the leuco dye.
A thermosensitive recording material, one of the above-mentioned recording materials, is usable as a recording material for an electronic computer, facsimile apparatus, ticket vending apparatus, label printer, and recorder because it has the advantages that complicated processes such as development and image-fixing are not required, recording can be achieved for a short period of time using a relatively simple apparatus, there is no noise development, and the manufacturing cost is low.
In such a thermosensitive recording material, colorless or light-colored leuco dyes having a lactone, lactam, or a spiropyran ring are used as coloring dyes, and organic acids or phenols are conventionally employed as color developers. The thermosensitive recording material using the above-mentioned leuco dye and color developer is widely used for practical use because the produced images have high density, with the whiteness of the background maintained high.
In line with the increase of demands for the thermo-sensitive recording system, the requirements for high speed recording are increasing. Therefore, not only the development of a high-speed recording apparatus, but also the development of a recording material capable of coping with the above-mentioned high-speed recording apparatus is intensively desired.
To cope with the high-speed recording system, it is proposed to use p-hydroxybenzoate and hydroxynaphthoate as the color developers with high sensitivity, as respectively disclosed in Japanese Laid-Open Patent Applications 56-144193 and 59-22793. However, the recording materials comprising the above color developers have a shortcoming in the preservability of the obtained images therein.
In addition, the use of a thioester compound as a color developer is proposed, as disclosed in Japanese Laid-Open Patent Application 59-165680; and phenolsulfonic acid and derivatives thereof are used as the color developers, as in Japanese Laid-Open Patent Applications 58-82788 and 60-13852. When the aforementioned compounds are used as the color developers, however, the sensitivity of the obtained thermosensitive recording materials is insufficient although image areas obtained in the recording materials are fast to fats and oils.
Furthermore, the addition of various thermofusible materials is proposed to increase the thermal sensitivity, for example, benzoate compounds in Japanese Laid-Open Patent Application 57-128592; phenylether derivatives in Japanese Laid-Open Patent Applications 58-57989, 58-87088 and 61-31287; naphthol derivatives in Japanese Laid-Open Patent Application 58-87064; and benzylbiphenyl in Japanese Laid-Open Patent Application 60-82382.
The thermosensitive recording materials comprising the above-mentioned color developers or thermofusible materials are not satisfactory from the viewpoints of the thermal coloring sensitivity, the whiteness degree of the background, and the preservability of the obtained images such as fading of the images and deposition of white dust on the recording material.
The use of a phenyl benzoate as a thermosensitivity-improving agent is proposed, as disclosed in Japanese Laid-Open Patent Applications 57-128592 and 58-59894. However, the thermosensitive recording materials comprising the phenyl benzoate as disclosed in the above applications are still insufficient in the thermal coloring sensitivity, and the preservability of the recording materials in terms of the image fading, deposition of white dust and coloring of the background.
The formation of an intermediate layer is proposed to increase the recording sensitivity without decreasing the whiteness degree of the background. For instance, there are proposed an intermediate layer mainly comprising minute void particles, as in Japanese Laid-Open Patent Applications 59-5093 and 59-225987; and an intermediate layer comprising non-expandable void particles, as in Japanese Laid-Open Patent Application 62-5886. However, these recording materials have the shortcomings that the thermal coloring sensitivity is insufficient and the recording properties are poor.